Errors using gtk.font_button in gtkada (gtk3)

[Gavin McCord <gavind.mccord@gmail.com>]

Strange one here, at least to me.

I'm running Slackware 14.2, using the XFCE 4.12
desktop environment and GNAT 2018 (though I've
encountered this problem with an older version
of GNAT also).

I created a small program "test" to take input
from a GEntry and insert it into a Text_View. It
doesn't do much else, but works okay.

Since then I've added a Font_Button to allow the
user to change the font within the Text_View.

I get an error though when I click on the font
button for the first time:

"(test:2564): Gtk-CRITICAL **:
gtk_tree_model_filter_real_unref_node: assertion
'elt->ref_count > 0' failed"

But the program, font selection included, still
works as expected.

Now all this is using a custom GTK3 theme, based
on the built-in Raleigh. If I then change the 
theme to Adwaita I get subsequent errors and
the program exits, as follows.

I start the program, and click the Font_Button.
The above error occurs, as before. At this point
I can still select a font and it changes the
text in the Text_View accordingly.

But if I then try to change the font a second
time, at the point when I click the Font_Button
the following additional errors appear and the
program exits:

"(test:3481): Gtk-CRITICAL **:
 gtk_list_store_get_value: assertion
 'iter_is_valid (iter, list_store)' failed

(test:3481): GLib-GObject-WARNING **:
 ../../src/gobject/gtype.c:4268: type id
 '0' is invalid

(test:3481): GLib-GObject-WARNING **:
 can't peek value table for type '<invalid>'
 which is not currently referenced

raised STORAGE_ERROR : stack overflow or
 erroneous memory access

I suspect this is not going to be easy to
diagnose as searches for these errors
previously hasn't brought me anything
particularly similar to my situation.

But any thoughts, pointers would
be gratefully received.

Gavin